Pressure in Fluids

by Ron Kurtus (revised 13 March 2016) - Appended by Anthony Kaburu (September 2016)

Pressure is a measurement of the force per unit [?]. Fluid pressure can be caused by pull of , acceleration, or forces in a closed container. A fluid is a state of matter that can [?] i.e. liquid or [?].

Since a fluid has no definite shape, its pressure applies in all directions. Fluid pressure can also be amplified through hydraulic mechanisms and changes with the velocity of the fluid.

Questions you may have include:

•How does gravity cause fluid pressure?
•How does air and water pressure apply in all directions?
•What are other applications of fluid pressure?

Fluid pressure from gravity or acceleration

The [?] of a fluid can exert a pressure on anything underneath it. Also, the relative movement of a liquid or gas can apply a pressure.

Pressure

Pressure is defined as [?] [?] by the [?] on which the force is [?]. (See the lesson on Pressure for details.) You can write this as an equation, if you wanted to make some calculations:

= /

where
•P =
• = e
• = [?]

Pressure due to gravity

Since the weight of an object or material is equal to the force it excerts due to , an object can exert downward pressure due to its [?] and the force of gravity. The pressure you exert on the floor is your [?] by the [?] of the soles of your shoes that is in contact with the floor. If the force is due to the weight (W) of the object, the equation is then: P = W / A

Water pressure

The water pressure at the bottom of a lake is equal to the weight of the column of water above divided by the area of that column.

Column on top of head

If you were standing on the bottom of a swimming pool (assuming you would not start floating), there would be a column of water the diameter of your head all the way up to the water surface, [?] down on you. If you took that column of water and weighed it, and then divided that weight by the area of the top of your head, you would get the value for the water on your head.

The reason it does not affect you is that your internal body pressure increases to neutralize most of the water pressure. But at greater depths, the water pressure can become so great that it can harm the diver.

To calculate pressure exerted by a liquid at a certain depth we use the folowing formula:



Demonstration with can

A demonstration of how water pressure [?] with the depth of the water can be done with a large tin can. Punch nail holes in a vertical line up the side of the can every inch or several centimeters. Then fill the can with water. The water may just dribble out the top holes, but the increased pressure with causes the water to squirt out with more pressure at the bottom holes.


At the same depth, the pressure exerted by water is the same. This is why a liquid column is used by technicians and carpeters to check if there is 'level' in what they are building: